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The present study shows by a linear hydrodynamic stability analysis that an unstable mixed–layer deep circulation can be generated in the dry convective well–mixed layer by the entrainment from the top. The newly–identified instability arises under the two competing processes induced by the top entrainment: the destabilization by generating thermal perturbations and the damping by mechanical mixing. The former and the latter, respectively, dominate over the other in the limits of large and small scales. As a result, the instability is realized at the horizontal scales larger than the order of the mixed–layer depth ( c.a. , 1 km), and the time scale for the growth is about one day. This study has been motivated from a question of whether the cloud–top entrainment instability (CTEI) can induce a transition of the stratocumulus–topped well-mixed boundary layer into trade-cumulus. The present study intends to extend the previous studies based on the local parcel analyses to a full analysis based on the hydrodynamics. Unfortunately, being based a dry formulation, the present result does not apply directly to the CTEI problem. Especially, the evaporative cooling is totally neglected. Nevertheless, the present result can still be applied to moist systems, to some extent, by re–defining certain terms in the formulation.

Well-Mixed Boundary Layer–Top Entrainment Instability: Hydrodynamic Analysis